At high altitude, it is typical for turbine engine combustors of auxiliary power units to have very low fuel flow rates. This is particularly true during low speed operation such as those which exist when the auxiliary power unit is being started even if all of the available fuel is directed through a pressure atomizing fuel injector of the most efficient type. For instance, even if an impingement injector is utilized, there is difficulty in achieving a satisfactory spray for ignition at low fuel flow rates.
In addition, this difficulty is still more severe with highly viscous fuels, and it is known that directing all of the fuel through a single start injector means considerable variations of air/fuel about the combustor. As a result, air loading is considerably higher, i.e., ignition and combustion are more difficult, than if fuel was uniformly introduced through all of the injectors.
Generally speaking, at typical low speeds air loading is very nearly linear with engine speed which means that at a given altitude ignition is extremely speed related. Thus, for instance, it's approximately twice as difficult to achieve ignition at 40,000 feet and at five percent speed from the kinetic viewpoint than at ten percent speed. As for altitude relatedness, and by way of example, at 50,000 feet it's nearly three times as difficult to achieve ignition at five percent speed from the kinetic viewpoint than at five percent speed at 40,000 feet.
Therefore, it is most desirable from an ignition reliability viewpoint to initiate ignition at the lowest possible engine speed. This is even more the case as altitude increases but, using conventional pressure atomizing injectors, even of the efficient impingement type, it is essentially impossible to achieve good atomization without gross overfueling. Of course, the overfueling is undesirable inasmuch as it also renders the air loading much higher than is desirable for reliable ignition.
Still further, as the engine accelerates, air loading increases rapidly up to a certain speed level. Thus, even if ignition can be achieved at, e.g., five percent speed, it is quite possible that flameout could occur at or near the air loading "worst case" operation level. By way of example, the conditions for burning may be over three times more difficult at the critical speed level.
In order to maximize altitude starting and operation, it is desirable to transfer to the main fuel at the lowest possible speed. This follows because the conditions for burning with a start injector will be worse than for the main fuel at the previously mentioned "worst case" or critical speed. However, this must also take into account the number of injectors for the main fuel relative to the total required fuel flow rate.
In addition, the fuel manifold head must be considered since it is necessary to maintain reasonable fuel distribution to all of the injectors for the main fuel so that air loading is not unduly increased. However, in so doing, the pressure drop required across the fuel metering orifices may become very large with the result that the fuel metering orifices may necessarily become very small.
It is well recognized that these can become critical parameters that require sensitive balancing to meet performance objectives.
The present invention is directed to overcoming one or more of the foregoing problems and achieving the resulting objectives.